app-server: expose loaded thread status via read/list and notifications (#11786)

Motivation
- Today, a newly connected client has no direct way to determine the
current runtime status of threads from read/list responses alone.
- This forces clients to infer state from transient events, which can
lead to stale or inconsistent UI when reconnecting or attaching late.

Changes
- Add `status` to `thread/read` responses.
- Add `statuses` to `thread/list` responses.
- Emit `thread/status/changed` notifications with `threadId` and the new
status.
- Track runtime status for all loaded threads and default unknown
threads to `idle`.
- Update protocol/docs/tests/schema fixtures for the revised API.

Testing
- Validated protocol API changes with automated protocol tests and
regenerated schema/type fixtures.
- Validated app-server behavior with unit and integration test suites,
including status transitions and notifications.

codex-rs/app-server/tests/suite/v2/thread_fork.rs

@@ -14,6 +14,7 @@ use codex_app_server_protocol::ThreadItem;
 use codex_app_server_protocol::ThreadStartParams;
 use codex_app_server_protocol::ThreadStartResponse;
 use codex_app_server_protocol::ThreadStartedNotification;
+use codex_app_server_protocol::ThreadStatus;
 use codex_app_server_protocol::TurnStatus;
 use codex_app_server_protocol::UserInput;
 use pretty_assertions::assert_eq;
@@ -80,6 +81,7 @@ async fn thread_fork_creates_new_thread_and_emits_started() -> Result<()> {
     assert_ne!(thread.id, conversation_id);
     assert_eq!(thread.preview, preview);
     assert_eq!(thread.model_provider, "mock_provider");
+    assert_eq!(thread.status, ThreadStatus::Idle);
     let thread_path = thread.path.clone().expect("thread path");
     assert!(thread_path.is_absolute());
     assert_ne!(thread_path, original_path);